It is a fundamental principle of linear optics that no system in which all the light enters from outside the system can ever have any trapped light. It is the common general knowledge of persons skilled in the art that light trapping requires that once light is generated within a system it must be totally internally reflected off all surfaces and that the basic reason why linear optical systems cannot trap light that has entered from the outside is the fact that if light passes in through a surface at a particular angle then light travelling in the exact opposite direction will pass out through that surface. This is because linear optical systems are time reversible (Pedrotti, F. L., and Pedrotti, L. S., Introduction to Optics, page 38. Practice Hall International, Inc, New Jersey, 1987.).
If light enters a light guide system from the outside and is reflected from the far end, it will tend to reach the entry surface with an angle similar to that with which it started, and will thus be able to pass through that surface and so escape from the system. This means that the system in which light enters from an external source can have no trapped light.
Only light sources that generate light inside a material can produce internally trapped light (Saleh, B. E. A. and Teich, M.C., Fundamentals of Photonics, pages 18, 39, and section 16.1. Wiley, 1991). Examples of internal light sources are fluorescent molecules and electrons and hole pairs in semiconductor light emitting diodes. A portion of the emission from such sources is often completely trapped within the system by total internal reflection (Saleh and Teich 1991).
The key point is that this light can travel on a closed path but such a path cannot be duplicated by light entering from the outside. Light entering from the outside and returning to the entry surface will always strike it at an angle that lies outside the total internal reflection cone of angles and thus exits from the system.
Another simple argument based on fundamental thermodynamics can be used to show that no type of linear optical system can have trapped light enter from the outside. If it were possible for a linear system to losslessly accumulate light produced externally, then the light would be continuously accumulated without limit and so the energy density inside would increase without limit. Such a system could be used to generate temperatures which were arbitrarily larger than the source temperature--a clear violation of the Second Law of Thermodynamics. Internal source systems do not suffer from this problem because the accumulation of trapped light changes the properties of the system in a way that prevents the endless accumulation of light. For example, fluorescent dyes become opaque to their own emissions at large enough photon densities due to non linear effects and so a system using these dyes will eventually cease to accumulate trapped light.
Luminescent solar concentrators (also called light receiving stacks) are of increasing interest because of their ability to contribute to the transmission of sunlight to the interior of buildings, owing in large part to their lower installation, running and maintenance costs over both conventional lighting systems and solar lighting systems that use tracking mirrors.
Luminescent solar concentrators contain at least one luminescent species capable of emitting luminescent radiation upon excitation by incident solar radiation. A large proportion of the emitted luminescent radiation is totally internally reflected by the surfaces of the medium from which the concentrator is fabricated and propagates inside the medium to the concentrator's end surfaces.
For example, in a luminescent solar concentrator comprising a flat rectangular sheet, light emitted by luminescent species at small angles to the planar axis of the sheet is totally internally reflected by the sheet's upper, lower and side surfaces and propagates to one end surface where it can escape. It is also apparent that light emitted by luminescent species nearly perpendicular to the sheet's planar axis quickly escapes through an upper, lower or side surface without undergoing total internal reflection.
However, some of the luminescent light emitted at intermediate angles to the sheet's planar axis is totally internally reflected by the sheet's upper, lower and side surfaces and propagates to one end surface where total internal reflection from the end surface causes it to reverse its path and be reflected back down the sheet. This light is completely trapped within the sheet and is unable to escape through any smooth surface of the sheet. For example, in a flat rectangular sheet of refractive index 1.5 surrounded on all sides by air, each of the sheet's six surfaces release approximately 12.7% of the luminescent radiation, and 23.6% of the luminescent radiation is trapped within the sheet. (Most of the trapped light is eventually removed by absorption by the luminescent molecular species or by scattering from defects.)
The prior art has not successfully provided a means by which this trapped light may be released from the conduit at a location where illumination is required.
Luminescent concentrator/conduit systems known in the prior art consist of a luminescent concentrator which is connected to a smooth, transparent optical conduit which is, in turn, connected to a luminaire (which may be no more than the end of the optical conduit). Luminescent radiation from the concentrator enters the conduit where it is channelled by means of total internal reflection to the luminaire which allows the light to escape from the system in the required directions(s). For efficient light transfer to occur from the concentrator to the conduit, and along the conduit, the cross sectional area of the conduit (which may change along its length) must never be smaller than the exit area of the concentrator.
It has been found by the present inventors that if the joint between the concentrator and the conduit has a mismatch in refractive indices (as will always occur with an air gap and may occur with some glued joints), then a substantial fraction of the luminescent radiation striking the joint is reflected away from the conduit, back into the concentrator. For many concentrator geometries, this light is unable to escape through any surface.
It is therefore an object of the present invention to provide an optical conduit that includes luminaire means through which such `trapped light` can exit the system in a useful manner. It is another object of the present invention to ensure that the luminescent concentrator and conduit are sufficiently closely coupled to enable the concentrator's `trapped light` to enter the optical conduit, where it will substantially increase the amount of light that reaches the luminaire means at the end of the conduit.